What is meant by Quasi-Static Process in Thermodynamics?

Quasi means nearly or almost. So quasi-static process means nearly static process or nearly stationary process, or a process which proceeds with extreme slowness.

Quasi-static process proceeds from one equilibrium state to another equilibrium state till the end of the process i.e., all the states passed during the process are all equilibrium states.

In each state the process deviates from the equilibrium state by a very small amount and immediately attains its equilibrium.

Let us consider a system consisting of a gas in a cylinder fitted with a piston. Let W be the weight kept on the piston which just balances the upward force exerted by the gas. The system is initially in the equilibrium state. During initial condition, let the properties be P₁ V₁ and t₁. If the weight is removed then there will be an unbalanced force between the system and the surroundings and the piston will move up till it hits the stops.

Then again the system comes to another equilibrium state described by the properties P₂, V₂, t₂ but the intermediate states passed through by the system are non-equilibrium states. 

Now if the single weight on the piston is made up of many very small pieces of weights and these weights are removed one by one, then the departure of the state of the system from the thermodynamic equilibrium state will be very small. So, every state passed through by the system will be an equilibrium state.

A Quasi-static process is also called as Reversible process and is represented on the PV-diagram by a continuous line.

In other words suppose when one weight say a is removed then the system will deviate from its equilibrium by a very small amount and immediately comes to an equilibrium state a and when weight b is removed, the system again deviates by a small amount and immediately comes to another equilib­rium state b. Thus when these small weights are removed one by one the system proceeds from one equilibrium state to another equilibrium state till end, such a process which passes through all the equilibrium states is known as quasistatic process.